Flood Watch 4, NWS Medford, OR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
936 AM PST FRI DEC 19 2014

.SHORT TERM...BEHIND A FRONT, SHOWERS WILL CONTINUE THIS MORNING
WITH DECREASING COVERAGE EXPECTED IN THE AFTERNOON. SHOWERS ARE
MAINLY LIGHT AND EXPECT SOME SNOW TO CONTINUE OVER THE HIGHER
TERRAIN FOR 4500 FT AND HIGHER. THIS WILL BRING SOME LIGHT SNOW
ACCUMULATIONS ON THE MOUNTAIN PASSES TODAY. THEN MODELS ARE ON
TRACK WITH AN EXTREMELY WET WARM FRONT MOVING INTO THE AREA
TONIGHT AND SATURDAY WITH A COLD FRONT GRADUALLY MOVING INTO THE
COAST SATURDAY EVENING INTO SUNDAY. MODELS ARE SHOWING THIS WILL
BRING WIDESPREAD HEAVY TO MODERATE PRECIPITATION TO THE AREA.

THE GFS AND SREF ARE SHOWING HIGH PRECIPITABLE  WATER VALUES OF
1.0 TO 1.5 INCHES MOVING INTO THE COAST WITH STRONG ONSHORE
WESTERLY FLOW ALOFT. THIS WILL BRING MODERATE TO HEAVY
PRECIPITATION TO MUCH OF THE AREA BEGINNING TONIGHT AND CONTINUING
THROUGH SATURDAY. HEAVY PRECIPITATION IS EXPECTED FOR THE COAST
AND SOUTHERN OREGON CASCADES WITH AMOUNTS OF 3 TO 5 INCHES
EXPECTED. MODERATE TO LOCALLY HEAVY PRECIPITATION IS ALSO EXPECTED FOR
WEST SIDE VALLEYS.

SNOW LEVELS ARE EXPECTED TO RISE FROM 4500 TO 5500 FT TONIGHT TO
7000 TO 9000 FT SATURDAY AFTERNOON AND EVENING. ONE CONCERN WITH
SNOW LEVELS IS WHETHER SNOW LEVELS MAY REMAIN BELOW 5000 FT INTO
SATURDAY MORNING, ESPECIALLY FOR EAST SIDE LOCATIONS. CURRENT
MODELS SHOW SNOW LEVELS REMAINING ABOVE VALLEY FLOORS WITH
MOUNTAINS ABOVE 4500 TO 5000 FT SEEING LIGHT SNOW ACCUMULATIONS.

HEAVY RAINFALL ALONG THE COAST AND INLAND INTO DOUGLAS COUNTY WILL
BRING SIGNIFICANT RIVER RISES AND MAY BRING THE POTENTIAL FOR SOME
RIVER FLOODING, MAINLY ALONG THE COQUILLE, AND FOR SMALL STREAM
RIVER FLOODING.

ALSO WITH THE FRONT, EXPECT SOME GUSTY WIND CONDITIONS ON SATURDAY,
MAINLY FOR COASTAL AREAS, THE MOUNTAINS, AND INTO THE EAST SIDE AS
WESTERLY FLOW INCREASES OVER THE AREA.

RAINFALL CONTINUES INTO SUNDAY BUT AS A HIGH PRESSURE RIDGE TO THE
SOUTHWEST BUILDS INTO THE AREA, EXPECT PRECIPITATION TO DECREASE
BY SUNDAY AFTERNOON INTO SUNDAY NIGHT.

&&

.AVIATION...BASED ON THE 19/00Z TAF CYCLE...
SHOWERS WILL CONTINUE ACROSS THE AREA TODAY, TAPER OFF THIS
AFTERNOON, AND THEN A WARM FRONT WILL PUSH IN THIS EVENING THROUGH
SATURDAY MORNING. A MIX OF VFR AND MVFR CONDITIONS WILL OCCUR THIS
MORNING IN SHOWERS, WITH MOSTLY OBSCURED HIGHER TERRAIN. EXPECT THIS
MIX OF MVFR AND VFR TO CONTINUE INTO THE AFTERNOON. TONIGHT, WITH THE
WARM FRONT, EXPECT PRIMARY MVFR AND IFR TO DEVELOP ALONG WITH TOTAL
TERRAIN OBSCURATION. BTL

&&

.MARINE...UPDATED 515 PM PST FRI DEC 19 2014... ISOLATED
THUNDERSTORMS WITH LOCALLY GUSTY WINDS WILL CONTINUE THIS MORNING,
PRIMARILY ALONG AND NEAR COAST AND SOUTH OF CAPE BLANCO. HEAVY WEST
SWELL WILL BUILD TODAY...PEAKING AROUND 20 FEET AT 18 SECONDS FRIDAY
EVENING. THE LONG PERIOD SWELL WILL REMAIN HEAVY THROUGH SATURDAY AS
ANOTHER ROUND OF GALE FORCE WINDS ARRIVES WITH A STRONG WARM FRONT.
SUNDAY INTO MONDAY THIS WARM FRONT WILL LINGER ALONG THE COAST NORTH
OF CAPE BLANCO AS SEAS SUBSIDE AND WINDS GRADUALLY DIMINISH. THE
NEXT FRONT IS EXPECTED TO ARRIVE FROM THE NORTHWEST ON WEDNESDAY.
MODERATE TO STRONG AND GUSTY NORTH WINDS ARE EXPECTED BEHIND IT
CHRISTMAS EVE INTO CHRISTMAS DAY.

&&

.PREV DISCUSSION... /ISSUED 712 AM PST FRI DEC 19 2014/

DISCUSSION...
A COLD FRONT IS PUSHING THROUGH THE FORECAST AREA THIS
MORNING...BRINGING WITH IT MODERATE TO OCCASIONALLY HEAVY
PRECIPITATION AND SOME MOUNTAIN AND EAST SIDE SNOWFALL AS SNOW
LEVELS DROP TO AROUND 4000 FEET THIS MORNING.

MODERATE SNOWFALL IS BEING OBSERVED ON WEBCAMS AT DIAMOND LAKE...AND
ALONG HIGHWAY 97 AT CHEMULT THIS MORNING. LATEST RADAR DATA SUGGESTS
THE SIGNIFICANT BAND OF SNOW OVER THE CASCADES AND HIGHWAY 97 SHOULD
DIMINISH SOME BEFORE SUNRISE. SNOW SHOWERS SHOULD CONTINUE OVER THE
CASCADES AT TIMES TODAY WITH A FEW MORE INCHES OF SNOW POSSIBLE.
ADDITIONALLY...THUNDERSTORMS ARE POSSIBLE OVER COOS AND WESTERN
DOUGLAS COUNTY THIS MORNING AS SOME STRIKES HAVE ALREADY BEEN
OBSERVED AT THE COAST THIS MORNING IN THE UNSTABLE AIR MASS BEHIND
THE FRONT.

ATTENTION TURNS TO AN ATMOSPHERIC RIVER EVENT THAT WILL BRING A
RELATIVELY LONG PERIOD OF MODERATE TO HEAVY PRECIPITATION TO THE
FORECAST AREA FROM SATURDAY THROUGH SUNDAY BEFORE THE PLUME OF
MOISTURE RETREATS TO THE NORTH. SNOW LEVELS WILL BE BETWEEN 4000
AND 5000 FEET WHEN PRECIPITATION BEGINS...SO MOUNTAIN PASSES
SHOULD GET A QUICK SHOT OF ACCUMULATING SNOWS LATE TONIGHT INTO
SATURDAY MORNING BEFORE SNOW LEVELS BUMP ABOVE 5000 FEET MID TO LATE
SATURDAY MORNING. THEREFORE...TRAVELERS WANTING TO AVOID SNOWFALL
OVER MOUNTAIN PASSES SHOULD WAIT UNTIL THE AFTERNOON TO TRAVEL.
HOWEVER...ALL TRAVELERS IN THE AREA WILL HAVE TO CONTEND WITH
MODERATE TO HEAVY RAIN ON SATURDAY AT LEAST. ADDITIONALLY...WINDS
WILL BE VERY GUSTY AT THE COAST AND EAST SIDE SATURDAY. 850 MB
WINDS ARE FROM THE SOUTHWEST...AND MODELS SUGGEST A 7-9 MB
PRESSURE GRADIENT FROM OTH- ACV...WHICH MAKES GUSTS TO 50 MPH SEEM
VERY REASONABLE. FUTURE SHIFTS MAY WANT TO BUMP THIS UP EVEN
FURTHER IF DATA SUPPORTS THIS. WE INCREASED WINDS NEAR SUMMER LAKE
FOR SATURDAY AFTERNOON AS 50-70KT 700MB WINDS TRAVERSE THAT AREA.
ADDITIONALLY...HAVE INCREASED SHASTA VALLEY WINDS SIMPLY DUE TO
THE STRONG SOUTH TO NORTH PRESSURE GRADIENT EVEN THOUGH THE 700MB
WINDS ARE FROM THE WEST.

THE HYDROLOGIC OUTLOOK COVERS THE HEAVY RAIN SITUATION WELL AT THIS
TIME. LATEST RIVER MODELS SUGGEST THE COQUILLE RIVER AT COQUILLE
REACHES ACTION STAGE...AND A FLOOD WATCH MAY BE NECESSARY IF FUTURE
RUNS INCREASE THIS FURTHER.  THE CURRENT FORECAST REFLECTS A BLEND
BETWEEN THE NAM AND GFS IN TERMS OF PRECIPITATION AMOUNTS...WHICH
BASICALLY IS 5 INCHES FOR THE OROGRAPHIC-FAVORED PORTIONS OF THE
COAST RANGE AND 5 INCHES FOR THE CASCADES FROM CRATER LAKE NORTH.
INTERESTINGLY...INTEGRATED VAPOR TRANSPORT...AN INDICATOR OF HOW
SIGNIFICANT A RAINMAKER THIS SYSTEM WILL BE...TRENDS HAVE DECREASED
SOMEWHAT OVER THE PAST FEW GFS ENSEMBLE RUNS...BUT STILL ARE
IMPRESSIVE WITH 5 STANDARD DEVIATIONS ABOVE NORMAL PER THE MOST
RECENT RUN.  SNOW LEVELS WILL PEAK AT 9000 FEET LATE SUNDAY...AND
MOST OF THE HEAVY PRECIPITATION WITH THE SYSTEM WILL FALL AS
RAIN...ENTERING THE RIVERS AND STREAMS.  THIS AGAIN SUGGESTS THAT
THERE WILL BE SOME HYDROLOGIC IMPACTS WITH THIS STORM.

CONDITIONS DRY OUT MONDAY INTO TUESDAY BEFORE A COLD SYSTEM DROPS
DOWN FROM THE NORTHWEST AND EXHIBITS QUITE DIFFERENT BEHAVIOR
THAN THE LAST SYSTEM...WITH SNOW LEVELS LOWERING SHARPLY AND THE
POTENTIAL FOR SNOW AS LOW AS 3000 FEET. THIS OF COURSE IS OF
IMPORTANCE TO ANYONE TRAVELING IN THE AREA OR HOPING FOR A WHITE
CHRISTMAS.

AVIATION...BASED ON THE 19/00Z TAF CYCLE...
SHOWERS WILL CONTINUE ACROSS THE AREA TODAY, TAPER OFF THIS
AFTERNOON, AND THEN A WARM FRONT WILL PUSH IN THIS EVENING THROUGH
SATURDAY MORNING. A MIX OF VFR AND MVFR CONDITIONS WILL OCCUR THIS
MORNING IN SHOWERS, WITH MOSTLY OBSCURED HIGHER TERRAIN. EXPECT THIS
MIX OF MVFR AND VFR TO CONTINUE INTO THE AFTERNOON. TONIGHT, WITH THE
WARM FRONT, EXPECT PRIMARY MVFR AND IFR TO DEVELOP ALONG WITH TOTAL
TERRAIN OBSCURATION. BTL

MARINE...UPDATED 830 AM PST FRI DEC 19 2014...HEAVY WEST SWELL WILL
BUILD TODAY...PEAKING AROUND 20 FEET AT 18 SECONDS THIS EVENING.
LONG PERIOD SWELL WILL REMAIN HEAVY THROUGH SATURDAY AS ANOTHER
ROUND OF GALE FORCE WINDS ARRIVES WITH A STRONG WARM FRONT. SUNDAY
INTO MONDAY...THE FRONT WILL LINGER ALONG THE COAST NORTH OF CAPE
BLANCO WHILE SEAS SUBSIDE AND WINDS GRADUALLY DIMINISH. THE NEXT
FRONT IS EXPECTED TO ARRIVE FROM THE NORTHWEST ON WEDNESDAY. EXPECT
MODERATE TO STRONG NORTH WINDS CHRISTMAS EVE INTO CHRISTMAS DAY.
-BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...HIGH SURF ADVISORY UNTIL 10 PM PST SATURDAY FOR
     ORZ021-022-615-618.

CA...NONE.

PACIFIC COASTAL WATERS...SMALL CRAFT ADVISORY FOR WINDS FROM 10 PM THIS EVENING TO 4 PM
     PST SATURDAY FOR PZZ350-356-376.
     HAZARDOUS SEAS WARNING UNTIL 10 PM PST SATURDAY FOR
     PZZ350-356-370-376.
     GALE WARNING FROM 4 AM TO 4 PM PST SATURDAY FOR
     PZZ350-356-370-376.
     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 10 AM PST THIS
     MORNING FOR PZZ350-356-370-376.
     SMALL CRAFT ADVISORY FOR WINDS FROM 10 PM THIS EVENING TO 4 AM
     PST SATURDAY FOR PZZ370.

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